It has been proposed in Japanese Patent Publication 63291770A to incorporate a continuously-variable-ratio transmission ("CVT") of the "variable-ratio-epicyclic" type in a power-assisted steering system. The customary components of such a CVT are an epicyclic gear set and a speed-varying component or "variator", the latter comprising an input member, and intermediate reaction member and an output member. By the "variable-ratio-epicyclic" type of CVT we mean the type in which the axes of rotation of all three of the elements of the variator are concentric, so that the total configuration of the variator bears some resemblance to a conventional geared epicyclic or planetary gear set. The present invention is to be distinguished from the embodiment of JP-A-63291770 principally in that in the system described in that prior publication, the CVT does not generate and transmit any power to assist the steering effort provided by the operator. Instead, the function of the CVT appears to be to vary the manual steering ratio, between the steering wheel and the steering rack to which it is connected, so that quite separate means to generate power assistance receive relatively strong signals at low vehicle speeds and weak ones at high speeds.
The present invention is also of course to be distinguished from the many prior automotive CVT's of concentric arrangement (of which U.S. Pat. No. 4,922,788 shows an example), by the fact that in those CVT's the sole source of motive input has been the prime mover of the automobile. In the position servo systems to which the present invention relates there must be two separate sources of such inputs. For example, in a power steering system a first and manual input is applied via the steering wheel and the CVT must be driven by a second source of motive power in order to contribute the power-assistance that is required.